The present invention pertains to a touch sensitive switch arrangement having an I.sup.2 L structure which is useful with battery-powered electronic apparatus such as electronic wristwatches.
Operation of electronic apparatus is taken out in various modes. One way is a touch sensitive switch which employs the human body as a means of electrical conduction and thus delivers either an ON or OFF signal depending upon whether two touch electrodes are bridged via the human body.
Employment of this type of the touch switch is limited on battery powered apparatus and not on commercial power utilizing apparatus. This is due to the fact that perfect assurance is not expected of electrical insulation between a primary winding and a secondary winding of a power transformer and it is not possible to avoid an electrical danger of the human body in case of the commercial power utilizing apparatus. However, in case of the battery-powered apparatus limits are usually placed on the capacity of the battery and thus this needs a considerable reduction in power consumption. To this end C-MOS transistors are very often employed in the design of circuit construction of the battery-powered apparatus. In addition, there has been proposed touch switches similarly comprised of C-MOS transistors.
More recently, attention has been directed to a low voltage operating and low power consuming I.sup.2 L (Integrated Injection Logic) circuit. With such an I.sup.2 L structure resistors of large area are eliminated and only four masks are needed in manufacturing because of its simplified gate structure. Moreover, it does not have any adverse effect on other bipolar elements incorporated within the same wafer.
Accordingly, it is an object of the present invention to provide a touch sensitive switch arrangement which is inexpensive and suited for large-scale production and more particularly implemented with I.sup.2 L circuit technology.
The following are the requirements needed to operate a touch sensitive switch circuit successfully.
First of all, a range of resistance values of the human body enabling switching functions normally extends 10K.OMEGA. through 10M.OMEGA.. This is dependent mainly upon personal differences, differences in contact positions with the human body and environmental factors such as temperature and humidity. It is, therefore, necessary to determine for all the resistances values whether the touch sensitive switch circuit is bridged by the human body. The touch sensitive circuit from fulfill the following requirements:
1. It shall not operate inadvertently even when receiving induced voltage (hum) from a commercial power source so forth.
2. Circuit damage shall be caused upon receipt of static electricity on the order of several thousand through twenty or thirty thousand volts.
3. It shall have response characteristics which will not degrade operating facility.
4. An operation range shall withstand differences in various factors of used components.
5. Switching voltage and switching current will not vary with accompanying a wider range of switching operation.
In accordance with the teachings of the present invention, there is provided an improved touch sensitive switch arrangement suited for I.sup.2 L implementations. This includes a battery power source, a current limiter, an injector and a first-stage and second-stage switching transistors being supplied with current from the injector. A first touch electrode is connected to the positive polarity of the battery power source while a second touch electrode is connected to an input terminal of an I.sup.2 L circuit. The first-stage switching transistor within the I.sup.2 L circuit structure is normally OFF to minimize power consumption. An operation range of the I.sup.2 L circuit is not limited by utilizing the above-mentioned connections.